The present invention relates to a color image forming apparatus in which: a color toner image is formed on a photoreceptor by means of electrophotography; the color toner image is transferred onto a transfer sheet; and the transferred image is thermally fixed to obtain a color image on the transfer sheet. More particularly, the present invention relates to a color image forming apparatus that can form a color image of high quality on one or both surfaces of a transfer sheet.
A large number of electrophotographic color image forming methods and apparatus are known. For example, the following color image forming method is disclosed in Japanese Published Application No. 100770/1986: Latent images, the number of which is the same as the number of separated colors, are formed and developed on the surface of a photoreceptor drum. Each time the latent image is developed, the image is transferred onto the surface of a transfer drum to form a multicolor image on the transfer drum. After that, the image is transferred onto a transfer paper to obtain a color image.
Another multicolor image forming method is disclosed in Japanese Published Application No. 149972/1986. According to the aforementioned method, a color image is formed as follows: Latent images, the number of which is the same as the number of separated colors, are formed and developed on the surface of a photoreceptor drum. Each time the latent image is developed, the image is transferred onto the surface of a transfer sheet to obtain a multicolor image.
Further, there is a color image forming method by which color images are formed in the following manner: Latent images, the number of which is the same as the number of separated colors, are formed and developed by color toners on the surface of a photoreceptor drum so that color toner images are superimposed on the photoreceptor drum.
The fundamental process of this multicolor image formation is disclosed in Japanese Published Application Nos. 75850/1985, 76766/1985, 95456/1985, 95458/1985 and 158475/1985.
In the case of a multicolor image forming apparatus that forms a color image by superimposing color toner images, operations are carried out to obtain a color image as follows: A plurality of developing units provided with toners of different colors are disposed around a photoreceptor drum. In general, the photoreceptor drum is rotated a plurality of times so that a latent image on the photoreceptor drum can be developed.
Concerning the image forming body, apart from the aforementioned photoreceptor drum, the circumferential surface of which is coated or vapor-deposited with a photoconductor, there is also a belt-shaped image forming body that is composed in such a manner that a flexible belt is coated or vapor-deposited with a photoconductor. This belt-shaped image forming body (referred to as a photoreceptor belt, hereinafter) can be provided between rotational rollers including a drive roller, so that space can be effectively utilized and a compact color image forming apparatus can be provided. Since the photoreceptor belt can be moved around a small radius of curvature, a transfer sheet can be separated from the photoreceptor belt utilizing a portion of the photoreceptor belt, the radius of curvature of which is small, wherein a rotational roller of a small diameter is used to form the portion of the small radius of curvature. Therefore, the occurrence of defective separation of a transfer sheet can be avoided.
Image formation is carried out by the apparatus shown in FIG. 19 to which the aforementioned photoreceptor belt is applied. FIG. 19 is a schematic illustration showing the structure of a conventional color image forming apparatus.
A photoreceptor belt 1 is wrapped around rotational rollers 2 and 3 and tension roller 5, and when the rotational roller 2 is driven in the arrowed direction, the photoreceptor belt 1 is conveyed clockwise in contact with guide member 4. Between a cleaning unit 13 (having a blade 13A and a cleaning roller 13B) on the right and a transfer unit 12 on the left, there are provided a charging unit 6, an exposure section composed of a laser writing system unit 7 (having a drive motor 7A, polygonal mirror 7B, f.theta. lens 7C, and mirrors 7D and 7E), and four developing units 8 to 11 accommodating four kinds of color developers. When charging, exposing and developing operations are conducting on the photoreceptor belt 1 four times, a color toner image is formed, and the formed color toner image is transferred onto a transfer sheet by the transfer unit 12. The transfer sheet is sent from sheet cassette 14 through sheet guide 15 and timing roller 17 when supply roller 16 is rotated. After the transfer operation has been completed, the photoreceptor belt 1 is cleaned by the cleaning unit 13, and the subsequent image is formed by the same process as described above. The image formed on the transfer sheet is fixed by a fixing roller 18, and then the transfer sheet is discharged onto a tray mounted on an upper cover 20 through a discharging roller 19.
In the manner described above, a full color image can be provided. The quality of this full color image greatly depends on the fixing operation.
A heat roller fixing system is most generally applied to the fixing process for toner images in electrophotography. In the recent electrophotographic process by which color images are formed, it is required to fuse and cool a toner image sufficiently in order to ensure color reproduction properties and gloss of the image. Therefore, the belt type fixing system, that can fuse and cool toner images better than the heat roller fixing system, is applied to the color image forming apparatus.
A thermal fixing unit in which toner images are fixed through a thermal belt is disclosed in Japanese Published Application No. 36342/1974. In this unit, a press roller is brought into contact with a heat roller through a belt, and when a transfer sheet, on which a toner image is formed, is pinched between the press roller and the thermal belt, the toner image is fixed.
Japanese Published Application No. 122665/1986 also discloses a fixing unit in which toner images are thermally fixed through a belt.
Further, Japanese Published Application No. 706331974/1986 also discloses a fixing unit in which the fixing operation is carried out as follows: Two endless belts are provided between a heat roller disposed at the entry side and a cooling roller disposed at the delivery side. Flat portions of both belts run in parallel, and a transfer sheet on which a toner image has been formed, is pinched by the two belts so that it is conveyed from the entry side to the delivery side.
In the same manner as the image formation of monocolor, color images can be formed on either surface of a transfer sheet, i.e. they can be formed on one side or on both sides of the transfer sheet.
In an automatic two-sided recording apparatus of the prior art, images are recorded on both sides of a transfer sheet in the following manner: After an image has been formed on one side of a transfer sheet in an image processing section, the transfer sheet is temporarily stored in a middle portion, and this transfer sheet is conveyed again to the image forming section so that an image is formed on the other side of the transfer sheet. As a result of the foregoing, images can be recorded on both sides of the transfer sheet.
As mentioned before, in the case of full color image formation, the fixing process has great influence on the quality of an image. For that reason, the belt type fixing system that can fuse and cool a toner image better than the heat roller type fixing system, has come into wide use for a color image forming apparatus.
As disclosed in Japanese Published Application No. 36342/1974, the fixing unit is provided with a press roller that is pressed against a heat roller through a belt, and a transfer sheet, on which a toner image has already been formed, is pinched between the press roller and the belt so that the toner image can be fixed. However, in the aforementioned structure of the prior art, not enough nip region can be provided, so that sufficient heat cannot be supplied from the upper heat roller through the belt. Accordingly, the toner cannot be fused sufficiently for the fixed image to become glossy.
In the case of the fixing unit disclosed in Japanese Published Application No. 122665/1986 in which fixing is carried out through a belt, nip region formed by the press roller and the belt is not sufficiently long, so that sufficient fusing time cannot be provided and the transfer sheet cannot be heated sufficiently. As a result, the fixing property is influenced by the material and thickness of the transfer sheet. Therefore, the thermal fixing operation becomes unstable.
In the case of the fixing unit disclosed in Japanese Published Application No. 70633/1874, the fixing operation is carried out as follows: Two endless belts are provided between a heat roller disposed at the entry side and a cooling roller disposed at the delivery side. Flat portions of both belts run in parallel, and a transfer sheet on which a toner image has been formed is pinched by the two belts so that it is conveyed from the entry side to the delivery side.
When the surfaces of the heat roller and press roller are hard, the endless belt can be regarded as hard. When a transfer sheet on which a toner image has been formed is conveyed between the rollers, contact pressure is given onto a very narrow portion. Therefore, the transfer sheet is fluctuated due to the vibration of the rollers. Accordingly, a different part of the transfer sheet experiences a different thermal expansion. As a result of the foregoing, the transfer sheet is wrinkled.
On the other hand, a conventional automatic two-sided recording apparatus is arranged in the following manner: After an image has been formed on one side of a transfer sheet in an image processing section, the transfer sheet is temporarily stored in a middle portion, and this transfer sheet is conveyed again to the image forming section so that an image is formed on the other side of the transfer sheet. As a result of the foregoing, images can be recorded on both sides of the transfer sheet.
As can be seen from the foregoing, the structure of the conventional automatic two-sided recording apparatus is complicated, because the passage for transfer sheets is long and complicated. Therefore, the conventional automatic two-sided recording apparatus is disadvantageous in that jamming of transfer sheets tends to occur. The aforementioned complicated passage is provided in the lower center of the main mechanism of the image forming apparatus, so that the entire structure becomes further complicated, and the cost is increased. Further, in the case of the occurrence of jamming, it is difficult to clear a jammed transfer sheet.
The present invention has been designed to solve the aforementioned problems. It is a primary object of the present invention to provide a simple color image forming apparatus of low cost that can easily record an image of high quality onto one side of a transfer sheet or both sides of a transfer sheet, and the image forming apparatus of the invention can form an image of high quality in such a manner that: the image quality is not affected by the material or thickness of a transfer sheet; wrinkles are not caused on the transfer sheet; and the occurrence of offset is avoided.